The present invention relates to a temperature controller (control system) for a fuel gas in a fuel cell system which generates an electric power by supplying a modified hydrocarbon gas as a fuel gas to a fuel cell.
A fuel cell system is an electric power system mainly composed of a fuel cell. The fuel cell generates an electric power by supplying hydrogen as a fuel gas to a hydrogen pole of the fuel cell and supplying an oxidizing gas containing oxygen gas such as air to an oxygen pole of the fuel cell. The fuel cell system directly converts a chemical energy to an electric energy and has a high power efficiency. In addition, the fuel cell system is assumed to be very clean power generating system which discharges negligible amount of environmental contaminating substances and, thus, has been under the examination of applicability to a wide variety of fields.
In such a fuel cell system, from the view points of difficulty to handle hydrogen and diversification of fuel sources, a gas except for hydrogen is utilized as a fuel gas. For example, a hydrocarbon fuel such as methane (CH4) or methanol (CH3OH) is reformed in a reformer to generate hydrogen, and the modified gas comprising hydrogen as a main ingredient is utilized in many cases. In the case of a vehicle like an automobile, on which fuel cell is carried, the use of hydrogen is inconvenient in the requirement of a long period of time for filling hydrogen and in the difficulty in carrying a large amount of hydrogen, resulting in a shortened mileage. For this reason, it has been considered that a liquid fuel like methanol is charged into an automobile, to be utilized as the fuel by modifying the liquid fuel in the reformer to generate a gas containing hydrogen as a main ingredient. Since methanol can be charged just like refueling and a mileage in this case is in no way to inferior to that in the case of the present automobile utilizing gasoline, making it possible to treating the automobile just like gasoline based car. What is more, in the case of utilizing methanol, since the methanol molecule only has one hydrogen atom, the amount of hydrogen generated is large and the proportion of carbon dioxide discharged is small in comparison with any other hydrocarbon fuel having a larger number of carbon atoms.
With reference to FIG. 5, the conventional fuel cell system 50 will be specifically described. The hydrocarbon fuel (methanol in this case) is introduced to a reformer 61 of a fuel gas generator 60 together with water and air at which the hydrocarbon fuel is modified to produce a fuel gas. While carbon monoxide (hereinafter sometimes referred to as xe2x80x9cCOxe2x80x9d ) is contained in the resulting fuel gas generated in the reformer only in a trace amount, CO, which poises the catalyst in a fuel cell 52, is converted into carbon dioxide by a CO remover 62 to be removed. The chemical reaction within the CO remover 62, of course, has an optimal temperature range. If the temperature is lower than this range, the proportion of converting (removing) CO becomes low, and conversely, if it is higher than this range, there is a possibility to bring about xe2x80x9cconverse shiftxe2x80x9d or xe2x80x9cmethanationxe2x80x9d where the hydrogen generated unduly undergoes oxidation.
Since the fuel gas generated in the reformer 61 has a high temperature (e.g., 300xc2x0 C.), the fuel gas is allowed to cool down to an appropriate temperature (e.g., 100xc2x0 C.) by means of a heat exchanger 71in at an inlet side, and then introduced into the carbon monoxide remover 62. The fuel gas from which CO has been removed by the CO remover 62 is introduced into the fuel cell 52. Since the chemical reaction within the carbon monoxide remover 62 is exothermic, the temperature of the fuel gas is increased (e.g., 180xc2x0 C.). On the other hand, the working temperature of the solid macromolecule fuel cell is from normal temperature to approximately 150xc2x0 C., a heat exchanger 71out at an outlet side is placed between the CO remover 62 and the fuel cell 52 to cool the fuel gas (e.g., cooled to 80xc2x0 C.). Subsequently, an electric powder is generated due to the reaction between the fuel gas supplied at the side of the hydrogen pole and air supplied at the side of the oxygen pole to supply electric power to a motor etc.
As described above, it is important to control the temperature of the fuel gas at the inlet and outlet of the carbon monoxide remover 61 in the fuel cell system 50, and the temperature is controlled by a temperature control system 70.
The temperature control system 70 has a circulating channel 76 for circulating a coolant medium (cooling water), having a radiator 72, a thermostat 73 for controlling the temperature of the coolant medium, a circulating pump 75 and the like in addition to the heat exchanger 71in at an inlet side and the heat exchanger 71out at an outlet side. In this temperature control system 70, the coolant medium circulating within the circulating channel 76 is controlled so as to keep its temperature at a constant level.
However, since the temperature control system 70 as described above has a configuration that the temperature of the coolant medium is controlled within a constant level by a thermostat 73, and the temperature of the fuel gas is controlled by the coolant medium having the constant temperature, the temperature of the fuel gas is decided basically by the ability of the heat exchanger 71, the temperature of the fuel gas at the inlet of the heat exchanger 71 and the flow amount. For this reason, the temperature of the fuel gas at the inlet of the CO remover 62 and that at the inlet of the fuel cell 52 cannot be controlled at a desirable level in a precision manner. Particularly, when the thermal load at the heat exchanger 71 is rapidly increased, as in the case where the temperature of the fuel gas is rapidly increased, the temperature control of the fuel gas utilizing the coolant medium having a constant temperature, has a restriction and, thus the temperature of the fuel gas at the outlet of the heat exchanger 71 is unduly increased. This problem cannot be solved if the flow amount of the coolant medium (cooling water) become variable.
Furthermore, in the case of carrying the fuel cell system on an automobile, since the fuel cell system is used in the state of a high variation in the thermal load of the fuel cell 52, it is required to keep the temperature of fuel gas at a constant level, quickly corresponding to the variation in the load according to driving operation.
An object of the present invention is, therefore, to provide a temperature control system for controlling a temperature of a fuel gas in a fuel cell system, which can quickly respond to the sharp variation in thermal load in an exchanger to control the temperature of the fuel gas to a desirable value in a precision manner.
We have studied to solve the problems associated with the prior art and to attain the object described above. As a result, we have found the fact that when a mechanism for controlling the temperature of the fuel gas is added to a thermostat for controlling the temperature by keeping the temperature of a coolant medium at a constant level, the temperature can be controlled at a desirable level in a precision manner due to the synergism between them, and completed the present invention.
The temperature control system for controlling a fuel gas of fuel cell of the present invention comprises a fuel reformer for reforming a hydrocarbon fuel into a reformed gas mainly comprising hydrogen, carbon monoxide remover or removing a carbon monoxide in the reforming gas and said reformed fuel gas is supplied to said fuel cell, said temperature control system comprising:
at least one heat exchanger which exchanges heat between the fuel gas and a coolant medium, said heat exchanger being placed at an inlet side and/or outlet side of said carbon monoxide remover,
a radiator which radiates the heat exchanged by said heat exchanger,
a thermostat which is connected to said radiator and a radiator bypass channel which bypasses said radiator, said thermostat being actuated by said coolant medium at a predetermined temperature, so as to decrease the flow of the coolant medium from the radiator when the temperature of the coolant medium is lower than said predetermined value, and increase the flow of the coolant medium from the radiator when the temperature of the coolant medium is higher than said predetermined value,
a thermostat bypass control valve connected to said radiator and said heat exchanger,
a control unit which detects the temperature of said fuel gas and/or said coolant medium, and controls said thermostat bypass control valve based on said detected temperature, so as to open said thermostat bypass control valve when said detected temperature is higher than a second predetermined temperature and, and to close said thermostat bypass control valve when said detected temperature is lower than said second predetermined temperature.
According to the temperature control system of the present invention, the thermostat bypass control valve is subjected to the coolant medium flowing within the thermostat to be bypassed to control the temperature of the fuel gas flowing toward the heat exchanger irrelevant to the temperature set by the thermostat. The temperature of the coolant medium flowing toward the heat exchanger is decided by the temperature of the coolant medium at the outlet of the thermostat, the temperature of the coolant medium at the outlet of the radiator, and the mixing ratio of both coolant media. The heat exchanger may be placed either in the inlet side or at the outlet side of the carbon monoxide remover, or at both sides of the carbon monoxide remover. If the heat exchanger is placed at the inlet side of the carbon monoxide remover, the temperature of the fuel gas at the inlet of the carbon monoxide remover is controlled. If the heat exchanger is placed at the outlet side of the carbon monoxide remover, the temperature of the fuel gas at the outlet of the carbon monoxide remover is controlled.
The xe2x80x9cthermostat bypass valvexe2x80x9d used herein is intended to include those which cannot control the opening of the valve (ON/OFF valve) as well as those which can control the opening of the valve in a voluntary manner. In the case of the thermostat bypass valve which can control the opening degree of the valve in a voluntary manner, the term xe2x80x9copening the thermostat bypass control valvexe2x80x9d used herein includes the operation of valve in such a manner that the flow of the coolant medium is increased at a voluntary proportion. Also, in such a thermostat bypass valve, the term xe2x80x9cclosing the thermostat bypass valvexe2x80x9d used herein includes the operation of valve in such a manner that the flow of the coolant medium is decreased at a voluntary proportion.
The term xe2x80x9cdetected temperature being within a prescribed levelxe2x80x9d used herein means that the detected temperature (preferably, the temperature of the fuel gas at the outlet of the heat exchanger) is within the temperature range tolerable for operating the CO remover or the fuel cell under the optimal temperature condition. The detected temperature may be the temperature of the fuel gas flowing towards the radiator or the temperature of the fuel gas flowing toward the heat exchanger or the temperature calculated from the combination of these temperatures by the control unit.
The temperature control system according to the present invention may be configured so that said temperature control system has the heat exchangers at the inlet and the outlet sides of the CO remover and, said circulating channel connects the heat exchangers in series to circulate the fuel gas from the heat exchanger placed at the outlet of the CO remover toward the heat exchanger placed at the inlet of the CO remover.
By such a configuration, the heat exchange between the coolant medium and the fuel gas is carried out in a counter flow.
The temperature control system according to the present invention may also be configured so that said temperature control system has the heat exchangers at the inlet and the outlet sides of the CO remover and, said circulating channel connects the heat exchangers in parallel.
According to this configuration, the temperatures of the fuel gas at the inlet and the outlet of the CO remover are controlled through own channels different from other channels of the heat exchangers. The differences in temperature xcex94t between the coolant medium and the fuel gas in all heat exchangers may be large.
In this configuration, the temperature control system may further be configured so that each heat exchanger has the thermostat, the thermostat bypass valve, and the circulating pump.
According to such a configuration, due to each heat exchanger having the thermostat, the thermostat bypass valve, and the circulating pump, the heat exchangers can control the temperature without any affect of the other heat exchanger.